Stents are radially expandable endoprostheses which are typically intravascular implants capable of being implanted transluminally and enlarged radially after being introduced percutaneously. They have also been implanted in urinary tracts and bile ducts. They are used to reinforce body vessels and to prevent restenosis following angioplasty in the vascular system. They may be self-expanding or expanded by an internal radial force, such as when mounted on a balloon.
Stents are generally tubular devices for insertion into tubular vessel regions. Balloon expandable stents require mounting over a balloon, positioning, and inflation of the balloon to expand the stent radially outward. Self-expanding stents expand into place when unconstrained, without requiring assistance from a balloon. A self-expanding stent is biased so as to expand upon release from the delivery catheter.
A vessel having a stenosis may be viewed as an inwardly protruding arcuate addition of hardened material to a cylindrical vessel wall, where the stenosed region presents a somewhat rigid body attached along, and to, the elastic wall. The stenosis presents resistance to any expansion of the vessel in the region bridged by the stenosis. Stenoses vary in composition, for example, in the degree of calcification, and therefore vary in properties as well.
The arcuate geometry of any stenoses present a variation in resistance along the vessel axis to stent outward radial force. Specifically, stenosed vessel resistance is often greatest toward the middle, lessening toward the ends, with a rapid decrease at the start of healthy vessel tissue.
In some instances, as in regions of bifurcation, stenoses are believed to be flow related phenomena, see Chapter 21 of the "Handbook of Bioengineering" (Richard Shaloh & Shu Chin, McGraw-Hill Book Company, 1987) which discusses atherosclerosis at vascular bifurcations.
The left and right common carotid arteries are typical of such vascular bifurcations. These arteries are the principal arteries of the head and neck. Both of the common carotid arteries are quite similar and divide at a carotid bifurcation or bulb into an external carotid artery and an internal carotid artery. In the region of the carotid bulb and the ostium of the internal carotid artery, stenoses present a particular problem for carotid stenting due to the large tapering of the vessel interior from the common carotid artery (both the left and the right) to the internal carotid artery. The region of the carotid bifurcation or bulb happens to be where stenoses most often occur, particularly in the region of the ostium to the internal carotid artery in both of the carotid arteries. Self-expanding stents are generally preferred for carotid stenting due to the anatomical location being subject to external compression.
A conventional self-expanding stent optimally has a length greater than the length of the stenosed region to be kept open. Current stents present a substantially uniform outward radial force and a uniform resistance to compression along their length. Currently, stents do not vary these forces to match vessel geometries or resistances. A constant force stent, i.e., prior art stents, with sufficient force to maintain an open channel within a stenosed vessel and to resist compression, has greater force than necessary in the healthy vessel portion distal to the stenosis. The stent end may thus flare outward, protruding into, and possibly irritating non-stenosed tissue.
Stenoses can occur in vessel regions having asymmetric geometry lying on either side of the stenosis. One example of this is the ostium of an internal carotid artery, having a wide opening converging into a narrower artery. A conventional stent placed in the region of the ostium would provide substantially uniform outward radial force over a non-uniform vessel diameter, that is, the force provided would be greater in a small diameter than in a larger diameter. If this force is properly matched for the smaller vessel region, it is likely less than optimal for the larger region. Conversely, if this force is properly matched for the larger vessel region, it is likely more than optimal for the smaller vessel region.
What would be desirable, and has not heretofore been provided, is a tapered stent capable of providing sufficient force to keep a vessel open within a rebounding stenosis, while providing only necessary force against healthy, non-stenosed vessel regions. What else has not been provided is a tapered stent providing necessary, but only necessary force (outward force and compression resistance) along a stenosis in a vessel region having non-uniform vessel diameter on either side of the stenosis. This is provided by the tapered stents of this invention which exhibit differing radial force, cell size, geometry, flexibility and which provide substantially more constant metal to artery ratio (M/A) over their length. M/A is the ratio of the metal surface area of a stent to the surface area of the vessel or the like that the stent is covering.